1. Field of the Invention
The invention relates to automatic control for steering systems of marine vessels, otherwise referred to as marine autopilots. The invention is particularly concerned with the integral channel of a PID autopilot with proportional, integral and derivative channels. Specifically, the invention relates to the rudder order bias integrator of such systems.
2. Description of the Prior Art
Marine autopilots are known in the art that provide automatic steering control of marine vessels. Such a system is disclosed in U.S. Pat. No. 4,074,648, issued Feb. 21, 1978, entitled "Adaptive Autopilot for Marine Vessels" by Reid and Wesner; U.S. Pat. No. 4,692,868, issued Sept. 8, 1987, entitled "Adaptive Autopilot" by Wesner et al; U.S. Pat. No. 4,777,602, issued Oct. 11, 1988, entitled "Digital Autopilot Controller for Marine Vessels", by Wesner; and U.S. Pat. No. 4,799,163, issued Jan. 17, 1989, entitled "Autopilot with Adaptive Weather Adjustment for Marine Vessels" by Wesner. Said U.S. Pat. Nos. 4,074,648, 4,692,868, 4,777,602, and 4,799,163 are assigned to the assignee of the present invention and are incorporated herein by reference.
Such systems include a rudder order bias integrator for removing heading keeping course offset. The rudder order bias integrator automatically computes a bias as a component of rudder order having the effect, in the steering control loop, of removing any bias that may be present in the heading error. Heading error bias results from non-zero average or mean moments acting on the ship. Such moments, which are due to wind and seaway effects or hull, propulsion, or cargo asymmetries, require a non-zero mean rudder position to maintain the ship on course. The bias generated by the integrator automatically provides the rudder correction required to maintain the mean heading aligned to the ordered course. Whenever the course is changed significantly, the bias integrator adapts to the new weather and seaway effects of the newly acquired course at sea.
In the prior art, during a course change, the bias integration function is disabled until the new course is achieved in order to prevent integration of heading error that is not associated with the non-zero mean hull moments. After achievement of the new course, the bias integration function is reactivated in order to begin computing the rudder order bias required on the new course. In the prior art, the bias integration function is reactivated after a fixed period of time, for example, 120 seconds, following the time that the heading error has diminished to within a specific value, for example, 10.degree..
The rate at which the prior art bias integrator corrects the non-zero mean moments is determined by a fixed value integration time constant. The integration time constant is selected large enough to prevent the bias integrator from responding to short term variations of heading error. A large integration time constant, however, results in a correspondingly long period of time for the bias integrator to achieve the required bias for a new course after a course change.
It has been observed on ships at sea that the bias integrator adapts so slowly after course changes that helmsman adjust the course selector of the autopilot for many minutes after completion of a turn in order to maintain the desired ship heading. It is undesirable that an automatic control system should require continuous manual adjustment, but this is the situation with prior art designs. The time that the prior art integrators require to detect the end of a maneuver to a new course and accordingly reactivate integration, as discussed above, increases the already long time for the bias integrator to settle out to the new output because of the long integration time constant. This combination of excessive time periods results in the long delay for the prior art integrator to finally reduce the mean heading error to zero after a course change. Typically, the helmsman adjusts the course selector during this period of time in order to "assist" the autopilot in quickly achieving and maintaining the ordered course.